1. Field of the Invention
The present invention relates to a heater control device for an air-fuel ratio sensor.
2. Description of the Related Art
To reduce both the fuel consumption and the amount of harmful gas emissions in an internal combustion automotive engine, the air-fuel ratio (A/F) of the mixture burned in the engine must be controlled over a wide range. To achieve such air/fuel control, an air-fuel ratio sensor having a sensor body constructed from an oxygen ion conducting element (sensor element), such as a zirconia solid electrolyte, with an electrode deposited on the reference atmosphere side and an electrode and a diffused resistor on the exhaust gas side, has been commercially implemented (this sensor is called a wide-range air-fuel ratio sensor or a linear air-fuel ratio sensor); this air-fuel ratio sensor detects the A/F ratio by utilizing the limiting current that is generated in proportion to the concentration of oxygen or unburned gas in the exhaust when a voltage is applied across the sensor body, and feedback control is performed based on the output of the air-fuel ratio sensor.
In performing air-fuel ratio feedback control based on the output of the wide-range air-fuel ratio sensor, it is indispensable to maintain the oxygen ion conducting element in an active state. For this purpose, control is performed to maintain the sensor element at a constant temperature by heating the element with a heater. At this time, the sensor element temperature needs to be detected but, since a correlation exists between the impedance and temperature of the sensor element, the need for a temperature sensor is eliminated as the sensor element temperature can be estimated by detecting the sensor element impedance (refer, for example, to Japanese Unexamined Patent Publication No. 10-232220 (corresponding U.S. Pat. No. 6,083,370)).
However, as the sensor element degrades over time, the element impedance becomes higher for the same element temperature. As a result, when the sensor element has degraded, if feedback control of heater supply power is performed so that the detected value of the element impedance matches the target impedance, the element temperature may rise even when there is no degradation in the output characteristic or response of the air-fuel ratio sensor, and the sensor can be damaged.
The present invention has been devised in view of the above problem, and an object of the invention is to provide a heater control device, for an air-fuel ratio sensor, that maintains the air-fuel ratio sensor in an active state by detecting the sensor element impedance of the air-fuel ratio sensor and feeding it back to control the power supplied to the heater wherein, even when the sensor element has degraded, damage to the heater or the sensor element is prevented by suppressing an excessive rise in the temperature of the heater and hence the sensor element.
To achieve the above object, according to the present invention, there is provided a heater control device for an air-fuel ratio sensor, comprising: feedback control means for controlling supply power to a heater, based on element impedance of the air-fuel ratio sensor, for heating the air-fuel ratio sensor; half-activation time detection means for detecting half-activation time of the air-fuel ratio sensor; and power limiting means for limiting, based on the half-activation time detected by the half-activation time detection means, the power that is feedback-controlled by the feedback control means.
In the thus configured heater control device for the air-fuel ratio sensor according to the present invention, the half-activation time, which reflects the degree of degradation of the sensor element, is detected before performing heater supply power control based on the feedback of the sensor element impedance, and the supply power at the time of feedback control is limited based on the thus detected half-activation time; this provides a reliable means to avoid situations where excessive power is supplied by feedback control to the sensor element whose impedance has increased due to degradation of the element, and the temperature of the heater and hence the sensor element rises excessively, resulting in damage to the heater or the sensor element.